NEC Express5800/A1080a, User Manual, Page: 338 / 696

Hardware
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RAID5
In RAID5, data is distributed to HDDs by striping and, at the same time, the parity (redundant
data)
is distributed to the HDDs. This mode is called "striping with distributed parity".
Each of stripe x, stripe x+1, and parity (x, x+1) created from stripe x and stripe x+1 is written onto
a
specific HDD. Accordingly, the total capacity assigned to the parity is just the same as the
capacity of a single HDD. If any one of the HDDs configuring a logical drive is defected, data is
still available with no problems.
RAID6
RAID 6 extends RAID 5 by adding an additional parity block (Q) created by different calculation
method such as weighting by some factor, and thus uses block-level striping with two parity
blocks distributed across all the member disks. This mode is called "striping with duplex and
distributed parity". Accordingly, the total capacity assigned to the parity is just the same as the
capacity of two HDDs. If two of the HDDs configuring a logical drive are defective, data is still
available.
HDD 1
RAID Controller
HDD 2 HDD 3
Stripe 1
Stripe 4
Stripe 2
Pality P(3,4)
Pality P(1,2)
Pality Q(3,4)
Pality P(5,6) Pality Q(5,6)
Stripe 5
HDD 4
Pality Q(1,2)
Stripe 3
Stripe 6
Pality Q(7,8)
Stripe 7 Stripe 8
Pality P(7,8)
HDD 1
RAID Controller
HDD 2 HDD 3
Stripe 1
Stripe 4
Stripe 2
Pality(3,4)
Pality(1,2)
Stripe 3
Pality(5,6) Stripe 5 Stripe 6